The present invention concerns soundboard apparatus and a method of forming such apparatus, and particularly where such apparatus is made of composite materials.
Soundboards for pianos, harpsichords and similar instruments, are conventionally made of spruce wood. This material exhibits the favourable ratio of high stiffness to low density required to develop sound quality and sound power. Such properties are generally understood by piano designers to be those that best respond to the input of vibration energy from the strings of the piano.
However, spruce wood is subject to dimensional change with temperature and humidity. These climatic factors often lead to splitting of the soundboard and lack of tuning stability in adverse climates.
A responsive piano soundboard will have multiple natural frequencies that lie close to or at the frequency of the note being sounded. In the bass register of pianos the frequency interval (spacing) between such natural frequency modes, called eigen frequencies, is proportionally wider than in the upper registers. However the number and proximity of natural frequencies in the bass register can be greatly influenced by the thickness and hence stiffness of the sound board and to a lesser extent the stiffness of the bridge, board mounting and ribs. The thinner and more flexible the board the more eigen frequencies exist close to the note being sounded and the lower the lowest frequency (the first eigen mode) of these natural frequencies. Thus the bass response of a piano with a thin flexible rib-less soundboard is potentially better than that with a heavily ribbed and thick board. Further advantage can be gained by using different thicknesses in different zones of the board, a slightly thicker board in those regions of the piano soundboard responding to higher frequencies while using minimal thickness in those zones responding to bass frequencies can be beneficial. The position and shape of responding zones can be determined by finite element modal analysis of the soundboard under conditions of vibration energy input.
Pianos with thin and less robust and therefore less durable spruce wood soundboards that are less able to withstand loading from the strings have generally been found to have shorter quality life span and poorer tuning stability although their acoustic performance initially may be superior.
Various recently developed Carbon Fibre and Kevlar type composite materials incorporating carbon fibre, graphenes, fullerenes, and sundry glass fibres embedded in a matrix of resinous material offer significantly higher stiffness, strength and favourable ratio of stiffness to specific gravity. Piano designers have therefore attempted to apply some of these composites in piano sound board building. Generally this has been without a successful outcome because builders have been constrained to use too thick material for strength reasons in resisting the downwards pressure of the strings on the soundboard. Extra thickness has been found to favour undesirable high harmonics which are not usually acceptable to piano artists.
The strings of a grand piano are normally and principally held in firm contact with a bridge cap by a change of angle of the string in the vertical plane where it traverses the bridge. This creates a downwards contact load by the string on the bridge that, with adequate angular change, and string tension ensures the string when struck from below does not lose contact with the bridge cap. Loss of contact force results in a buzzing sound and inefficient vibration energy transfer to the sound board. This down bearing load is typically about 4 to 8 lbs (1.8 to 3.6 Kg) per string. As a consequence, a piano with about 230 strings will need a sound board stiff and strong enough to support a down bearing load of between 900 and 1 800 lbs (405-81 OKg). This defines the thickness and reinforcement means of the soundboard design needed to carry the down bearing load continuously throughout the many years of life of the instrument. A typical piano soundboard of adequate strength in spruce wood material is made of 4 to 9 mm thick wood planks butt joined and reinforced on its underside by rectangular often near square section spruce belly bars (ribs) typically about 25 mm in square cross section spaced about 1 00 mm apart.
To develop comparable stiffness and strength using, say carbon fibre would require a board material thickness of about 4 mm. Such large thickness in carbon fibre has been found to favour high frequency response and thus accentuation of undesirable upper harmonic sound. The applicants of the present invention have determined that the sound quality of a piano with such board is not generally acceptable.
Further it is well established that few if any spruce wooden boards last longer than 5 to 40 years in this duty before the sound power and quality of the instrument is lost as the board collapses under the continuous down bearing load, and sufficient contact force between string and bridge cap is reduced or completely lost. The thinner and thus weaker the board, the shorter the life of the instrument before this happens.
In the applicant's earlier patent disclosure No. WO 201 0 086690 A2, entitled “String-bridge interface system and method”, there is described a means of developing contact force between string and bridge cap without resorting to a change of angle in the vertical plane of the string where it traverses the bridge cap and thus without generating a down bearing force.